1. Field of the Invention
The invention relates to methods for forming or shaping a gear part, in which a workpiece is fixed with a radially directed area in a flow-forming machine, the workpiece is rotated and for modifying a wall thickness of the workpiece at least one roller is substantially radially infed. The invention also relates to a gear part, more particularly manufacturable with the methods according to the invention.
2. Description of the Background
Methods in which from a starting workpiece with a uniform wall thickness gear parts with a material-reinforced area are produced by flow-forming are well known. The aim of such methods is in a simple manner to produce by flow-forming a gear part, which on the one hand has an adequate stability and on the other a minimum weight.
These requirements more particularly apply to a starter ring gear for motor vehicles. As a result of its function a starter ring gear must at times transfer relatively high torques from a starter to an engine shaft. For relieving the starter a minimum starter ring gear weight is desirable.
The object of the invention is to provide methods and a gear part, particularly a starter ring gear, which more particularly satisfy the aforementioned requirements.
According to a first aspect of the invention a method according to the above-noted object is further developed in that the roller is radially moved along the radially directed area and workpiece material is radially displaced, the wall thickness of the workpiece being reduced in the particular area, and that for modifying the wall thickness of the workpiece the roller is axially adjusted during radial moving. In this manner, in a single method step, particularly during a single passage of the roller over the workpiece, a planned material distribution is set at the workpiece. The contour of the workpiece in its radial area can be made variable, because this is determined with respect to the workpiece by the cooperation of the radial and axial relative movement of the roller. By a thickness decrease or increase of the workpiece corresponding to the loading, a virtually optimum weight utilization is achieved on the finished gear part.
It can be advantageous for the axial infeeding of the roller to take place at least in part continuously. This leads to a smooth surface contour of the workpiece which can be advantageous as a function of the intended use of the gear part. It can also be useful if the axial infeeding of the roller takes place, at least in part, discontinuously. Thus, steps or other shapes can be made in the surf ace of the gear part, so that there is a considerable flexibility with regards to the design of the end product.
Preferably the wall thickness radially inwardly decreases in a radially inner area of the workpiece. Thus, the outer edge or rim of the circular blank maintains its original thickness and consequently all its material is available for further processing, particularly for inserting teeth. The inner area acquires in a planned manner a sufficiently great degree of flexibility, which is very advantageous in operation for avoiding wear on the tooth surfaces and for reducing noise formation.
Preferably the shaping of the boss is brought about through the shape of the roller and/or a countertool. Thus, different shapes can be given to the boss by selecting suitable tools.
It can also lead to advantages if, following its construction, the boss is cut off or punched. This can be useful with regards to the further processing of the workpiece, namely if processing steps are chosen in which the boss would be prejudicial.
The method according to the present invention is further developed according to a second aspect of the invention in that a radially outer edge of the workpiece is initially upset and cross-sectionally thickened in tear-shaped manner and following onto the tear-shaped, the thickened edge is upset and worked to a cross-sectionally approximately rectangular and in particular approximately square marginal area.
A fundamental idea of the invention is to shape the edge of a substantially radially extending workpiece so as to have a material accumulation, which has in an axial direction a predetermined width, as well as at least rounded comer areas, which link axially and radially directed lateral areas of the tear-shaped upsetting. In the following working step the rounded, tear-shaped edge or marginal thickening is slightly radially upset, while the axial width remains substantially unchanged. Corresponding to an angular contour of the spinning chuck or working roller during said working step the material displaced during upsetting is spun in the comer areas. This means that during this method step working and material flow almost exclusively take place in the lateral and in particular the comer areas, whereas the central area of the tear-shaped thickening is not deformed or is only scarcely deformed. Corresponding to the strong working or forming in the comer areas, an increased work hardening occurs there.
Thus, a marginally thickened workpiece is created, which firstly has a high strength comer and edge areas and secondly a less hardened tough core area. Such workpieces are eminently suitable for gear parts, because in these it is desirable to have a marginal area with a high strength, wear-resistant surface with a tough workpiece core suitable for dynamic loading. The material characteristics of the workpiece permit the manufacture of gear parts using conventional metal working steels with a high stability and a very low weight to form the gear parts.
Preferably use is made of a forming roller for the approximately rectangular forming or the circular blank edge, which has two radially projecting, shaping side walls, which enclose an angle between 1.5 and 6xc2x0. As a result of said slight bevel of the forming roller, after the upsetting process the roller can easily be removed from the formed workpiece.
It has proved to be particularly advantageous for the side walls of the forming roller A to have an angle of 3xc2x0 with respect to the radial direction.
Preferably the at least one forming roller is infed in an axially displaced manner with respect to the circular blank face. This leads to a gear part asymmetrical to the plane of centers, which is desirable in numerous applications.
The forming bevels resulting from the forming operation are preferably trued. Such forming bevels e.g. arise through the use of a forming roller with sloping side walls. As a result of the following truing of the resulting bevels it is possible to provide an exact, e.g. rectangular shape of the circular blank edge.
Preferably external teeth are formed in the circular blank edge. In the case of tooth systems it is desirable to have dynamically loadable teeth with high strength edge areas.
According to the invention it is appropriate for modifications to existing installations for the external teeth to be made by machining. This can e.g. take place using gear cutters or on corresponding slotting machines.
However, according to a particularly advantageous embodiment of the method according to the invention the external teeth are produced by flow-forming with at least one gear cutting roller or by cross-rolling with at least one cross-rolling ledge. As a result of this non-cutting forming of the tooth system a further work hardening occurs and also the tooth flanks are hardened with respect to the core areas of the individual teeth. In the case of gear wheels manufactured in this manner, even without a subsequent heat treatment excellent tribological characteristics of the tooth system are obtained. As a result of the highly strengthened edge and corner areas a premature tooth breakage or pitting are avoided in operation, even in the case of an unforeseen edge run of the tooth system.
It can also be advantageous if the tooth system is obtained by stacking several gear parts and subsequently making a tooth system in the gear part stack. It is consequently possible to simultaneously process several gear parts with a uniform operating process.
The invention also makes available a gear part which has a flat, rotationally symmetrical shape, the marginal area being cross-sectionally substantially thickened in rectangular manner and a radially inner area is radially inwardly reduced in thickness. Such a gear part, which can in particular be manufactured using the method according to the invention, with respect to the wear which occurs and the noise formation offers advantages compared with the prior art gear parts. Due to the reduction in the wall thickness the gear part is flexible, so that the teeth of the gear part combine in careful manner with the teeth of other gear parts.
The gear part is preferably provided in a central area with a boss. Thus, there is an inner area for the absorption or delivery of power and an outer area for the absorption or delivery of power.
It is particularly advantageous if the boss and/or the marginal area has a tooth system. These standard power transmission means are preferred in numerous applications.
The invention is based on the unpredictable finding that it is possible to reduce wear and noise formation during the operation of a gear part by giving the latter a certain flexibility. The gear part according to the invention can be manufactured in a simple and variable manner by the methods given, so that the advantages of the invention arise both during the manufacture and the use of the gear part.